The invention relates to a Penning type gauge head with an electrode system comprising an anode and a cathode disposed in the magnetic field of a permanent magnet, as well as with an ignition aid for the electrode system.
Penning type gauge heads of this type are installed in ionization vacuum gauges. They comprise two unheated electrodes (cathode and anode) between which, by means of a DC voltage of the order of magnitude of 2 kV, a cold discharge is ignited and maintained. With the aid of a magnetic field the path of the electrons is extended so that their impact rates with gas molecules is sufficiently large in order to form the required number of charge carriers for maintaining the discharge even at very low pressures. The positive and the negative charge carriers migrate to the corresponding electrodes and form the pressure-dependent discharge current which is displayed on a measuring scale. Of disadvantage in vacuum gauges of this type is that the discharge cannot always be ignited.
From DE-OS 30 47 744 a gauge head of the above described type is known. To attain a reliable ignition it is suggested to provide, apart from the measuring system comprising unheated electrodes, a glow cathode in addition. This measure is expensive since the supply of a sufficiently large current to the glow cathode is required. An additional current supply device as well as a separate current lead-in are required.
From EU-A-271 769 a gauge head of the type relevant here is known, which, apart from the measuring system (main measuring system), comprises a further Penning system with cathode and anode (ignition system). Its dimensions are significantly smaller than the dimensions of the main system. The production of an ignition system of this type is also expensive. Moreover, tests have found that the ignition of the measuring system is not always ensured. The gauge head known from EU-A-271 769 was therefore never introduced on the market.